Works are performed in an oil or gas well to stimulate or treat the well, whereby the production is increased, to replace various equipment such as valves, to make measurements, to monitor the state of the well, or anything else being required.
Treatment of the well to increase the production rate or volume is made after a cost/benefit evaluation. Even if the production from a well may be increased by several factors, the intervention costs may become too high or the work considered too difficult and time consuming. For onshore or platform wells, having easy access into the Christmas tree and infrastructure in the form of lifting equipment etc., the costs of performing the well intervention will be less relatively to the benefit of the operations. An intervention of subsea wells is much more expensive. A vessel (drilling rig or the like) has to be used, involving large daily expenses and, in addition, time consuming transit to and from the field, and large costs as the work is much more time consuming. Because of this, the production volume from a platform or onshore well is up to twice the volume of a subsea well with similar reservoir conditions. As mentioned above, this is caused by the more easy access making a better programme for well maintenance practically possible and profitable.
A well intervention may be difficult, as existing barriers have to be removed before entering the well. There are strict rules regarding which measures being required to prevent an uncontrolled blowout during such works. Thus, when well intervention shall be performed, a provisional pressure barrier has be established in the form of a blowout preventer. Depending on the work to be performed, this may vary from simple stop valves to large drilling BOPs.